FEATURE
Soil testing—first step to fertility boost Good soil testing is the foundation for getting the basics right for good soil management, and maximising crop or pasture yields. By Richard Rennie The main impediment to improve our farming productivity on New Zealand farms (both dairy and dry stock) is the ability to do the basics well, and this starts with fertility. We have discovered and documented which plants need which minerals and elements to grow to optimum health. Through our understanding of the periodic table, we are able to pinpoint specific elements (e.g. manganese, copper, boron, etc.) that are important to plants in trace amounts. We have developed fertilizers to replace the major elements (macronutrients) plants need for basic growth— Nitrogen, Phosphorus, and Potassium (N-P-K). Use of fertiliser is vital, not only to individuals farming our land, but also to the well-being of New Zealand as a whole. Under use of fertiliser that provides these key elements means landowners are not making the best use of the resources available. It is important to measure the existing soil nutrient status to assess whether a farm is in a development or maintenance phase. Soil testing, and taking into account fertiliser history, is the only way to do this. The following soil tests are available from most commercial laboratories: • PH—a measure of soil acidity and hence a test for lime requirement • Olsen P—a measure of plant available P. • Quick Test K (QTK)—a measure of plant available K. • Quick Test Mg (QTMg)—a measure of plant available Mg. • Quick Test Ca (QTCa)—a measure of plant available Ca. • Sulphate-S (SO4-S)—a measure of the immediately plant available S. • Organic-S (Org-S) or Total S—a measure of the long-term supply of S. • Anion Storage Capacity (ASC)—a measure of the capacity of a soil to store nutrients such as P and S (previously referred to as phosphate retention). • Cation storage capacity (CSC)—a measure of the capacity of a soil to store nutrients such as Ca, Mg, K and Na (also referred to as cation exchange capacity). However as simple as soil testing may sound, it can be made more confusing by the array of tests farmers can choose from when trying to determine how many thousands of dollars to spend each year on the most effective fertiliser applications. Because fertiliser is the major item of discretionary expenditure on the farm, soil fertility should be monitored regularly. However, soil tests, like all biological measurements, are variable and therefore a single soil test taken at one time is of limited value.
In the normal farm situation, soil sampling should be undertaken at least once every two to three years. Taking samples six to eight weeks prior to fertiliser application will allow the results of laboratory testing to be used to decide what and how much fertiliser should be purchased. Maximum advantage from soil analysis will be achieved by repeated testing over a number of years. In this way, a picture of trends in soil fertility status of the farm is built up. The advent of inexpensive, hand held GPS (Global Positioning System) units has meant that permanently recording the sampling lines (transects) is made easy. This allows soil samples in future years to be collected from the same sampling lines which helps reduce spatial variability of soil test information. Soil tests are unreliable for assessing trace element status because they are present in small quantities in the soil making the relationship between soil content and plant and animal requirements hard to define. Pasture analysis is essential to assess trace element status, and a useful back up to soil tests to confirm soil nutrient status. When trace-element deficiencies have been identified by herbage and/or animal liver tissue (or body fluid) analyses, they may be corrected by the addition of the required mineral (s) to the normal fertiliser application. Alternatively, some trace elements can be directly administered to animals. Some tests for certain elements can be unreliable in terms of the information they provide. Mineral N tests for example can be notoriously variable across paddocks, and down the soil profile. N tests are best used as a guide only. The tried and true Olsen P test continues to provide a consistent benchmark indicator of P levels. Canterbury soils typically enjoy high reserves of potassium (K) minerals, so demand for potassium fertiliser will depend on the types of crop being grown and the farming operation. Unlike their North Island counterpart’s dairying on volcanic soils, farmers in Canterbury dairy farmers may go many years before requiring fertiliser K. TBK tests for K availability need to however be done at every soil test, particularly in dairy and dairy support situations If there are any one problematic mineral in Canterbury and much of the South Island, it is sulphur (S), a problem former North Island dairy farmers are less familiar with. Sulphur analysis can be tricky to get right but Organic S is tested for in soil tests and is the base on which S is required. Sulphur is a critical element for rye and clover growth. Lime is important and must be used to get the PH levels into 5.8 to 6.2 zone, as our pasture species
(ryegrass and white clover) have been bred to perform best in this range). The simplest problem to correct is soil pH, and thanks to a plentiful supply of soft lime in Canterbury it is also one of the cheaper problems to fix. Typically cropping soils will require a pH similar to pasture, ideally between 5.8-6.2, although some crops like maize will tolerate lower pH readings and legume seed crops slightly higher pH readings. The use and rate of nitrogen used on crops and pastures will tend to lower the pH quicker. The old “one ton to the acre” (2.5t/ha) rule for two to three yearly application is still as valid as ever for pasture when pH levels need adjusting. Thanks to their friable, stony nature most Canterbury soils respond relatively quickly to lime applications. Alternatively, applied as a maintenance dressing lime need only be applied at 300-500kgs per ha to keep PH at optimum levels. Changes in land use in Canterbury are being reflected in changes to some soils’ physical structure. Soils are experiencing a build up of organic matter on lighter irrigated soils that have had dairy cows on them for 8–10 years. Wintering dairy cows on traditional arable farms has also bought some issues around soil compaction (pugging) and drainage, often requiring regular deep ripping to help rectify. Whatever situation faced if cropping, winter cultivation and fallowing is the worst option, as this leads to greater N losses. Similarly early spring application of N to crops too young to take it up can lead to increased N losses.
How to test: •
Sample areas that reflect the farm’s variability ie hill vs flats, terraces, north facing versus south facing slopes, cropping vs pasture areas.
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Use soil maps, aerial maps/Google Earth, EM scans to identify variances on the property.
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Sampling may involve sampling most paddocks or major zones every 3 years or one third every year. On large farms one paddock may represent several similar paddocks. On cropping farms paddocks may represent the major crops being grown, with some being annually tested and others less regularly.
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Sample pasture 0–7.5 cm, cropping soils 0–15 cm, in moist but not overly wet conditions, avoiding where stock or fertiliser have been recently.
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GPS transects can help identify sample areas for future reference.
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